1. Field of the Invention
This invention relates to optical beam amplification and more specifically to amplification and wavefront compensation of depolarized optical beams.
2. Description of the Related Art
A double-pass master-oscillator power amplifier (MOPA) used in conjunction with a phase conjugator (for wavefront compensation) is a well known approach for amplifying an optical beam from a low-power, diffraction-limited laser, while maintaining the beam's diffraction-limited divergence and linear polarization. In a double-pass MOPA, an optical seed beam from a diffraction-limited master oscillator, such as a solid-state laser, is passed through an amplifying medium twice.
Some of the amplifiers used in MOPAs are low-gain amplifiers due to the use of an inherently low-gain material (such as Nd:glass) as the gain medium. Even when a higher gain medium is used, such as neodymium-doped yttrium aluminum garnet (Nd:YAG), the possibility of thermal fracture due to overheating limits the pumping rate, and hence the gain, to low values. The double-pass architecture is not practical for use in conjunction with low-gain amplifiers because two passes through a low-gain amplifier will only amplify the beam by a relatively small amount, resulting in low amplifier extraction efficiency.
Higher amplifier extraction efficiencies can be achieved with low-gain amplifiers by increasing the number of passes that the optical seed beam makes through the amplifier. A four-pass architecture is described in N. F. Andreev, et al., "Multipass Amplifier with Full Utilization of the Active Element Aperture", Soviet Journal of Quantum Electronics, vol. 13, no. 5, May 1993, pages 641-643. Eight-pass architectures are described in C. B. Dane, et al., "Long Pulse Regenerative Amplifier Architecture with Diffraction-Limited Output Divergence", Conference on Lasers and Electro-Optics, Optical Society of America Technical Digest Series, vol. 11, May 1993, pages 274-275, and also in M. E. Brodov, et al., "Eight-Pass Neodynium Glass Slab Amplifier with a Waveguide and with Phase Conjugation", Soviet Journal of Quantum Electronics, vol. 17, no. 10, October 1987, pages 1265-1266. In both of these architectures, as well as other multi-pass architectures, the optical techniques used to couple the seed beam in and out of the amplifier chain require a linearly polarized seed beam.
There are applications in which it is desirable to amplify a depolarized seed beam. For example, laser-based avionic systems typically require high power diffraction-limited laser beams. When installing a high power laser system in an aircraft, it is often desirable to locate the emitting aperture near the nose of the aircraft or out on a wing. Using traditional lasers, system designers were forced to locate the entire laser system at the desired location, creating weight and volume distribution problems. One way to overcome this problem is to use multimode optical fibers to deliver the high power beam to the desired emitting aperture.
The high power beam can be obtained by amplifying a diffraction-limited seed beam with a MOPA, using a system similar to that described in U.S. Pat. No. 5,208,699, entitled "COMPENSATED, SBS-FREE OPTICAL BEAM AMPLIFICATION AND DELIVERY APPARATUS AND METHOD", issued May 4, 1993 to David A. Rockwell and John L. Bartelt and assigned to Hughes Aircraft Company, the assignee of the present invention. In this type of system the seed beam laser, the optical amplifier and a phase conjugator are located at a central station. The low-power seed beam is delivered to a local station, such as an emitting aperture located at the nose of an aircraft, through a single-mode, polarization preserving reference optical fiber. The seed beam is transmitted back through an optical fiber bundle to the central station for amplification and phase conjugation, and the amplified, phase conjugated beam is delivered back to the local station through the fiber bundle.
The fiber bundle used to deliver the seed beam to the central station and the amplified beam back to the local station must be multi-mode to accommodate the high-power amplified beam. The multi-mode fiber bundle depolarizes and aberrates the seed beam on its way from the local station to the optical amplifier. Since the amplified seed beam is phase conjugated by a phase conjugator, the depolarization and aberrations imposed on the beam by the multi-mode fiber are compensated when the phase conjugated, amplified beam passes back through the fiber bundle on its way back to the local station. However, the depolarization of the seed beam limits the number of passes through the optical amplifier to only two. This is because the depolarization of the beam is not compensated until after the amplified beam is returned to the local station through the fiber bundle. Since the beam is depolarized while it is at the central station, current four and eight-pass MOPA architectures cannot be used.